The subject development relates generally to the field of radiochemistry and more particularly to an instrument calibration standard for applications employing iodine-125, such as radioimmunoassay.
Radioimmunoassay (RIA) combines the sensitivity of radiochemistry with the specificity of immunology to produce a unique new diagnostic and research tool. As such, it has become an important aspect of analyses performed in diagnostic laboratories. Typical of the analyses are those related to hormones, proteins, drugs (both therapeutic and addictive), poisons, metabolites and vitamins.
The basic principle of RIA utilizes the specific antigen-antibody reaction. A known amount of antibody, present as a limiting factor, is mixed with a sample containing the antigen to be tested and a known amount of radioactive antigen. After a suitable incubation time, an equilibrium mixture of labeled (radioactive) and unlabeled antigenantibody complex is formed. Since both endogenous and labeled antigen compete for the active sites of the antibody, the amount of labeled complex formed is a function of (inversely proportional to) the antigen concentration of the sample. More details about RIA principles may be found in a review article by D. S. Skelley, et al., in Clinical Chemistry, 19, 146 (1973).
A number of radioactive isotopes are potentially useful in RIA applications. These include tritium (.sup.3 H), iodine-131, iodine-125, carbon-14, cobalt-57, and cobalt-60. The two most common radioisotopes are .sup.3 H and .sup.125 I which are utilized in more than 90% of all commercially prepared RIA products. Of these, .sup.125 I is now most often utilized.
As stated above, a determination of the labeled complex must be made in RIA. The measurement of this complex containing a gammaemitting isotope such as .sup.125 I can be, and most often is, accomplished by transferring the sample into a tube which is placed in a cavity (well) in a scintillator crystal such as thallium-activated sodium iodide. The emissions from the isotope are thereby converted to light pulses which, in turn, are received by a photomultiplier tube to give rise to an electrical signal proportional to the energy absorbed from the radioactivity present in the sample. If the detecting unit is a spectrometertype instrument, the range of energy of specific interest may be studied. In the case of .sup.125 I, for example, this range is from about 0.017 MeV to about 0.075 MeV because of the x-ray emission at an energy of about 0.028 MeV, a .gamma.-ray emission at an energy of about 0.035 MeV and (in a well counter) a sum coincidence peak at about 0.063 MeV. The two lower peaks of the spectrum normally are observed as a single broad peak well separated from the sum coincidence peak.
As in any radioactivity determination, periodic checking of the measuring equipment is necessary. For this purpose a "standard" is utilized to ascertain proper operation. In the case of .sup.131 I, where the half-life (time to decay to one-half of the activity) is eight days, a stand-in or "mock .sup.131 I" was developed having a very long half-life, as disclosed in U.S. Pat. No. 2,831,122. In the case of .sup.125 I, the iodine isotope .sup.129 I has been utilized; however, this does not adequately ascertain all of the operating conditions of the instrumentation because the sum coincidence peak is missing.